Recently, wavelength-multiplexing optical communication systems, where signal lights with different wavelengths are propagated on an optical transmission line to increase quantity of information to be transmitted through the optical transmission line, are increasingly developed as means for realizing a large-capacity optical communication system.
In general, for a long-distance transmission, a variation of transmission level between channels occurs because of a wavelength dependency in transmission loss of the transmission line. Further, when multistage repeating is conducted by using some optical repeaters, a difference of transmission level between signal wavelengths occurs because of a wavelength dependency in the gain of the repeaters. Optical repeaters typically include an optical fiber amplifier. Therefore, the optical fiber amplifier needs to have an equalized wavelength dependency in its gain characteristic so as to conduct a desired optical transmission. Thus, in wavelength multiplexing optical transmission, the equalization of optical level is important.
Conventionally developed techniques for gain equalization of optical amplifier are as follows.
The first technique is a method of equalizing gain by using P/Al co-doped EDF (erbium doped fiber) and Al-doped EDF suggested in Kakui et al., "Hybrid EDFA(erbium doped fiber amplifier) Module for WDM(wavelength division multiplex) Transmission", Institute of Electrical Communication, Society Meeting, B-1094(1996). The second technique is providing a gain equalizer with an etalon filter in an optical amplifier as suggested in Okuno et al., "Optical Fiber Amplifier Module and Gain Equalizer for WDM Transmission", EDM96-42. The third technique is a method of equalizing gain by using fluoride-doped EDF suggested in Yamada et al., "Wide Band and Gain Equalized Type Er.sup.3+ -doped Fluoride Optical Fiber Amplifier", Institute of Electrical Communication, Electronics Society, C-221(1995).
However, the first to third techniques for gain equalization have the problems described below.
Namely, the paper to suggest the first technique for gain equalization reports that a gain deviation of 1.5 dB is obtained. Thus, it does not meet a gain deviation of less than 1.0 dB which is desired in WDM transmission.
In the second technique for gain equalization, the theoretical gain deviation is controlled to be 0 dB and the paper reports that a gain deviation of less than 1.0 dB can be stably obtained. However, it cannot adjust a gain to a specific wavelength since the wavelength characteristic of the gain equalizer is fixed.
Also, in the third technique for gain equalization, a gain deviation of less than 1.0 dB is obtained. However, the reliability of a fluoride-doped EDF is so terrible as compared with that of a normal EDF. Therefore, it is not commercially available.
Meanwhile, all the first to third techniques are concerned only with an optical fiber amplifier. Thus, they cannot serve as a solution for the variation in transmission level between channels caused by a wavelength dependency in transmission lose of a transmission line in case of a long-distance transmission.